The present invention relates to generation of X-rays for medical purposes, and in particular it relates to miniature X-ray source for intravascular treatment of lesions in body tissue, in particular for treatment of stenosis in coronary vessels and treatment of cancer tumors.
Radiation therapy is a well-established method for treatment of several diseases, including cancer. The presumptive usefulness of a miniature X-ray source is clear. The insertion of such a source into vessels or other body cavities would allow the delivered dose to be confined to a small tissue region. More specific, a catheter with a miniaturized X-ray source could be used for irradiation of cardiovascular tissue. A stenosed coronary artery is often treated by balloon dilatation, i.e. Percutaneous Transluminal Coronary Angioplasty (PTCA). A small balloon at the top of a plastic catheter is inserted into the femoral artery, guided in the vessels to the site of stenosis, and inflated. As the stenosis is pushed out by the balloon the artery is widened to normal inner diameter. However, in about one third of the patients a restenosis will occur after the PTCA. One means of reducing the restenosis rate is to treat the vessel wall locally with gamma or beta radiation in conjunction with the PTCA. An absorbed dose of about 10-50 Gy from catheterised gamma and beta sources has been shown to lower the restenosis rate substantially in several trials.
In contrast to radioactive sources the here suggested X-ray source would be switchable on and off electronically. Furthermore, the energy of its radiation would be given by the voltage between the electrodes. As the range of X-rays in tissue depends on the X-ray energy the optimal range could be obtained by applying the corresponding electrode voltage.
In applicants own U.S. patent application Ser. No. 08/805,296 (corresponding to WO 98/36796), incorporated herein in its entirety by reference, there is disclosed a miniaturized source If ionizing electromagnetic radiation, comprising a pair of plates; a hermetically sealed microcaoity formed in one of the plates; a pair of electrodes in the form of a cathode and an anode, at least one electrode being located in the microcavity and the other electrode being located on the other plate; the anode being at least partly of a metal having a relatively high atomic weight; and electrically conducting leads connected to the cathode and the anode.
The cathode of the above device is preferably provided as a pointed tip of a material such as oxides of metals from group II in the periodic table, including cesium, barium and magnesium.
Although representing a major step forward in the art of radiation sources, the disclosed device has certain shortcomings.
In view of the cathode being shaped as a pointed tip the current density attainable will not be very high, and hence the number of electrons impinging on the anode will be relatively small, and thus the generated X-ray intensity will not be very high. In a therapy situation this would lead to longer treatment times, which is undesirable.
U.S. Pat. No. 5,854,822 discloses an X-ray device having a cold cathode. The cathode is composed of a mixture of a diamond powder and a getter, and has the general shape of a rotationally symmetrical cone. This patent specifically discloses that diamond powder based cathodes yield better performance that diamond coated cathodes.
WO 98/48899 discloses the use of microscopic metal tips, such as of Mo, together with a gate electrode for the purpose of controlling the electron emission. Thereby high fields are achieved locally. In addition a specific X-ray transmission window is used in order to achieve sufficiently short treatment times. The provision of a gate electrode and the addition of a specific X-ray transmission window adds to the complexity of the structure, which is cumbersome and expensive considering the small dimensions. In particular the contacting and electrical control of the gates is difficult.
Thus, there exists a need for miniaturized radiation devices for therapeutic applications, which are capable of generating X-ray radiation of an intensity of a desired magnitude.
Therefore the object of the present invention is to improve the prior art devices in order to meet the above requirements.
This object is achieved with a miniaturized X-ray radiation source as defined in claim 1.
Thereby the cathode is provided as a layer comprising diamond or carbon based material having diamond like structure and exhibiting a plurality of tips. Examples are polycrystalline diamond, diamond like material (by which we i.a. mean boron nitride and silicon carbide, and other structurally similar materials), amorphous diamond and diamond like carbon. The layer can be deposited onto a suitable substrate, having the desired geometry.
The tips may be formed by depositing a continuous/covering polycrystalline layer of diamond, in which case they will be in the nanometer range, or they may be made by depositing diamond in recesses made by an isotropic etching in single crystal silicon to provide xe2x80x9ctemplatesxe2x80x9d for pyramidally shaped tips. Once the diamond is deposited in said recesses, enough of the matrix silicon is etched away to leave the pyramids exposed, extending but from the silicon base. It is also possible to employ isotropic etching, whereby the shape of the tips will be more quasi-spheroidal or egg-shaped.